1. Field of the Invention
The present invention relates to a reduction casting method and reduction casting apparatus in which casting is performed while an oxide film formed on a surface of molten metal is reduced and, further, a molding die for use in an aluminum reduction casting method.
2. Description of the Related Art
There are various types of casting methods, but a gravity casting method has many advantages such as a favorable quality of a cast product, a simplicity of a molding die and the like. FIG. 5 shows an example of a molding die for use in casting aluminum by the gravity casting method. The molding die 100 is made of metal and has a split-type constitution including a lower mold 102a and an upper mold 102b. These two molds 102a and 102b form a cavity 104 in which a cast product having a desired shape is cast.
In the upper mold 102b, a feeder head portion 108 is formed between a sprue 106 from which a molten metal of aluminum, an alloy thereof or the like is poured and the cavity 104, and also an air-vent hole 110 is formed for discharging an air present in the cavity 104 at the time the molten metal is poured into the cavity 104.
When the molten metal is solidified, shrinkage of about 3% is generated. For this feature, the shrinkage generated by solidifying the molten metal poured in the cavity 104 appears as a defect such as a shrinkage hole or the like in an obtained cast product. When the molten metal filled in the cavity 104 is shrunk as being solidified, the feeder head portion 108 arranged in the molding die 100 shown in FIG. 5 replenishes the molten metal into the cavity 104 by a force of gravity to prevent the defect such as the shrinkage hole or the like from being generated. Since such a replenishing action of the molten metal from the feeder head portion 108 to the cavity 104 is performed by a force of gravity of the molten metal filled in the feeder head portion 108, a conventional casting apparatus secures a large capacity as the feeder head portion 108.
This is because, since a flowing property of the molten metal in the molding die in the casting apparatus is low, it is necessary to allow a weight of the feeder head portion 108 to be large thereby forcibly replenishing the molten metal into the cavity 104. For example, in a case that aluminum is cast, since aluminum is extremely easily oxidized, there is a problem that an aluminum oxide film is formed on a surface of the molten metal to decrease the flowing property of the molten metal. For this reason, a coating agent which aims for enhancing the flowing property of the molten metal is sometimes applied on a surface of an inner wall of the cavity 104.
With reference to such a method of casting aluminum as described above, the present applicant has proposed (in Japanese Patent Laid-Open No. 280063/2000) an aluminum casting method which can enhance the flowing property of aluminum without using the coating agent to obtain an aluminum cast product having a favorable outward appearance. This aluminum casting method, as shown in FIG. 6, is characterized in that, after magnesium-nitrogen compound (Mg3N2) being a reducing compound, is introduced into the cavity 104 of the molding die 100, molten metal of aluminum or an alloy thereof is poured into the cavity 104 to be cast. The magnesium-nitrogen compound has an action to reduce an oxide film formed on a surface of the molten metal of aluminum or the alloy thereof and, by this action, a surface tension of the molten metal is decreased to enhance the flowing property and a running property of the molten metal and to eliminate a surface fold and the like whereupon high-quality casting can be performed.
In the gravity casting method, in order to prevent air or an oxide from being entrained at the time of filling the molten metal in the cavity, the molten metal is filled in the cavity by allowing it to be in a state of a laminar flow. In order to fill the molten metal in the cavity in a state of the laminar flow, in a conventional molding die, a gate which connects the sprue and the cavity is allowed to be large whereupon the molten metal is poured into the cavity from a lower surface thereof such that a surface of the molten metal is gradually raised to prevent a turbulent flow from being generated as much as possible. The reason for allowing a diameter of the feeder head portion 108 to be large in the molding die 100 according to FIG. 5 is that an action of the feeder head by the molten metal in the feeder head portion 108 is secured and entrainment of the air or an oxide is prevented as much as possible at the time the molten metal is poured into the cavity 104. Further, in order to pour the molten metal in a state of the laminar flow, a method of pouring the molten metal while the molding die is tilted has widely been used.
As described above, in the gravity casting method, there is a problem that, since the gate is allowed to be large to prevent the turbulent flow from being generated at the time of pouring the molten metal and there is a restriction that the gate is arranged in a position where pouring the molten metal is easily performed by the laminar flow, a degree of freedom of the molding die or the apparatus is regulated. Further, there is a problem that the apparatus becomes large and complicated in a case in which a tilting-type molten metal pouring operation is performed. Furthermore, the yield by the conventional gravity casting method is ordinarily from 50% to 60%, which is hardly favorable in comparison with other casting methods.
The present invention is attained in order to solve such problems of the conventional gravity casting method as described above and has an object to provide a high-quality and efficient casting method by utilizing a reduction casting method which performs casting while an oxide film formed on a surface of the molten metal is reduced by making use of the above-described reducing compound. In a case of the reduction casting method, since the oxide film formed on the surface of the molten metal is reduced, a flowing property of the molten metal is enhanced and a running property thereof is improved whereby the filling property of the molten metal in the cavity becomes favorable. The present invention is to provide a reduction casting method which enables an action of such a reduction method as described above to be more effectively exerted, a reduction casting apparatus and a molding die advantageous to an aluminum reduction casting method.
In order to achieve the above-described object of the present invention, constitutions described below are provided.
Namely, according to the present invention, there is provided a reduction casting method, in which molten metal is poured into a cavity of a molding die and casting is performed while an oxide film formed on a surface of the molten metal is reduced by allowing the molten metal and a reducing compound to be contacted with each other in the cavity of the molding die, comprising the step of:
pouring the molten metal into the cavity while it is allowed to be in a turbulent flow in the cavity at the time the molten metal is poured into the cavity.
Further, according to the present invention, there is provided a reduction casting method, in which molten metal is poured into a cavity of a molding die and casting is performed while an oxide film formed on a surface of the molten metal is reduced by allowing the molten metal and a reducing compound to be contacted with each other in the cavity of the molding die, comprising the steps of:
arranging a runner having a smaller flow passage diameter than that of a feeder head portion in an upstream side of the cavity; and
adjusting a flow rate of the molten metal to be poured into the cavity by adjusting the flow passage diameter of the runner.
Further, according to the prevent invention, casting is performed while molten aluminum or a molten alloy thereof is used as the molten metal and a magnesium-nitrogen compound, which is obtained by introducing a magnesium gas and a nitrogen gas into the cavity and, then, allowing the magnesium gas and the nitrogen gas to be reacted with each other therein, is used as the reducing compound.
Further, according to the present invention, there is provided a reduction casting apparatus, in which molten metal is poured into a cavity of a molding die and casting is performed while an oxide film formed on a surface of the molten metal is reduced by allowing the molten metal and a reducing compound to be contacted with each other in the cavity of the molding die, comprising a runner having a smaller flow passage diameter than that of a feeder head portion arranged in an upstream side of the cavity.
Further, according to the present invention, the feeder head portion is arranged just upstream of the cavity, and the runner is connected with the feeder head portion.
Further, according to the present invention, a molten metal reservoir for storing the molten metal is arranged at a sprue which is arranged in an upstream side of the runner, and an opening/closing member for opening/closing a communication between the molten metal reservoir and the runner is arranged. By these arrangements, the molten metal stored in the molten metal reservoir can be poured into the cavity at a time; on this occasion, the molten metal can be poured into the cavity with an increased flow rate.
Further, according to the present invention, a surface of an inner wall of the runner is subjected to a heat insulating treatment or formed by a heat insulating material selected from the group consisting of: ceramic, an alumina board and other heat insulating materials. By this arrangement, a flowing property of the molten metal in the runner becomes favorable whereby the flow rate of the molten metal at the time of being poured into the cavity can be increased.
Further, according to the present invention, there is provided a molding die for use in an aluminum reduction casting method, in which molten metal of aluminum or an alloy thereof is poured into a cavity and casting is performed while an oxide film formed on a surface of the molten metal is reduced by allowing a magnesium-nitrogen compound which is prepared by allowing a magnesium gas and a nitrogen gas to be reacted with each other and the molten metal to be contacted with each other in the cavity, wherein a runner having a smaller flow passage diameter than that of a feeder head portion is arranged in an upstream side of the cavity.